The invention relates to an illumination unit, comprising:
a high-frequency supply having a first and a second output for supplying a high-frequency varying voltage relative to ground, said voltages having the same magnitude and being in phase opposition, said high-frequency supply being provided with a further output which is substantially free of high-frequency voltage variations relative to ground; and PA1 a low-pressure discharge lamp having a tubular discharge vessel with an ionizable filling, which discharge vessel comprises a first and a second external electrode, which are arranged at opposite ends of the discharge vessel and which are connected to, respectively, the first and the second output of the supply. PA1 a high-frequency supply having a first and a second output for supplying a high frequency varying voltage relative to ground, the voltages having the same magnitude and being in phase opposition, the high-frequency supply being provided with a further output which is substantially free of high-frequency voltage variations relative to ground; and PA1 a low-pressure discharge lamp, having a tubular discharge vessel with an ionizable filling, which discharge vessel comprises a first and a second external electrode, which are arranged on either side of the discharge vessel and which are connected to, respectively, the first and the second output of the supply; PA1 the discharge vessel of the low-pressure discharge lamp comprising a further external electrode which is centrally arranged between the ends of the discharge vessel and which is connected to the further output of the supply.
The invention also relates to a liquid crystal display device comprising such an illumination unit.
Such an illumination unit is disclosed in "Frequency Dependence of RF-Driven Subminiature Fluorescent Lamps", Alexandrovich et al., J. of the Ill. Eng. Soc., Winter 1996, pp. 93-99. The lamp described in this document is a low-pressure mercury discharge lamp having a length of 20 cm and an internal diameter of 5.2 mm. A supply having a first and a second output terminal whose voltages have the same magnitude and are in phase opposition, will hereinafter also be referred to as a symmetrical supply. By using a symmetrical supply, interference is counteracted. Such illumination units are widely used as a backlight in liquid crystal display devices. Since the lamp is operated by means of external electrodes, a long service life can be achieved. A sufficiently efficient operation of such lamps requires, however, a high operating frequency, for example, of the order of one MHz and higher. In the known illumination unit, the operating frequency is 13.56, 27.12 or 40.68 MHz. It has been found that at high operating frequencies, deviations from a uniform brightness distribution readily occur as a result of parasitic capacitances between the discharge vessel and neighboring conductors. However, for applications in a display device, a uniform brightness distribution across the discharge vessel is very important.